1. Field of the Invention
This invention relates to display devices. In particular the invention relates to display devices including an array of switchable elements, each switchable element being switchable between at least two states, the form of the image displayed by the display device being dependent on which state each switchable element is in.
2. Description of the Prior Art
Such switchable elements may take the form of light modulators which control the passage of light from a light source to the displayed image. Examples of light modulators include deflectable mirror devices as for example described in "Deformable Mirror Spatial Light Modulators" by Hornbeck, published in the Proceedings of SPIE, Vol. 1150, August 1989. Such deflectable or "deformable" mirror display devices include an array of deflectable mirror devices, each mirror device being mounted on a torsion element over a control electrode. Applying an electric field between each mirror device and the electrode causes the mirror device to pivot, thus changing the direction of light reflected from the mirror device.
Another example of a light modulator is a liquid crystal device.
Alternatively, the array of switchable elements may take the form of an array of light sources which themselves can be switched either on or off, for example, an array of light emitting diodes.
Generally such display devices are digital devices, that is each switchable element of the device is effective to switch the light passing from the element to the displayed image either "on" or "off" so as to produce either "white" or "black" pixels on the displayed image. It is, however., possible to display grey scale images by controlling the time for which each switchable element of the device is in a state such that light from the element arrives at the displayed image, and using the integrating response of the human eye of an observer who will perceive a grey scale image from the element.
An example of such an arrangement is described in GB 2014822 which discloses a display device incorporating an X-Y array of energizable light emitting devices. The display device described in GB 2014822 takes data in binary digital forms, for example via an 8 bit signal, the device being driven a line at a time in a number of periods during which the modulators may be "on" or "off". The "on"/"off" state of each pixel during each time period is determined by the state of the corresponding bit of the digital input data.
Display devices incorporating light modulators in the form of deformable mirror devices operate in an analogous manner. In deformable mirror devices, however, the entire pixel array is driven simultaneously in sympathy with the video source vertical scan rate.
The eight time periods within each display frame period are of different lengths. The length of the time period corresponding to the least significant bit (LSB) in the input signal for any particular frame is set at a predetermined value, the duration of the time period corresponding to the next to the least significant bit being twice as long as that corresponding to the least significant bit, and so on. Thus, the length of the time period corresponding to the most significant bit (MSB) for such an eight bit input signal is 128 times that corresponding to the least significant bit (LSB). Provided that all the time periods are included within a display frame period of less than around 20 msecs duration, the human eye will integrate the periods and respond as if to a single period having a level of brightness corresponding to the binary signal value. At the end of each sub-frame period corresponding to a single bit of the input signal, a resetting signal is supplied to all the elements of the array in order to switch the elements into a rest position in some systems, or into the state determined by the next bit signal.
Such display systems, using time division multiplex addressing of the switching elements at least partially to display grey scales, are used in large screen projectors. It is found however that where such large screen projectors are used to display motion video signals, there are formed bands of twinkling pixels at certain mid-grey brightness levels. Furthermore, for the same mid-grey brightness levels, the displayed image can break up into "chunky" pixels if a viewer blinks his eyes, moves his head, or waves his fingers in front of his eyes or in front of the projection lens. These effects are sometimes known as "dynamic contouring".